FIG. 1 (Prior Art) is a simplified block diagram of a so-called “motion detector” or “motion sensor” as might be a part of a common home security system. The motion detector 1 detects motion by detecting infrared radiation emitted by a person. For example, the face of a person emits a substantial amount of infrared radiation that usually can be detected at a reasonable distance under normal lighting and temperature conditions by a passive infrared (PIR) sensor 2. PIR sensor 2 is also referred to as a pyroelectric sensor. A multi-section lens 3 is placed in front of the detector. Lens 3 has a plurality of lens sections. Each lens section directs infrared radiation from a corresponding respective zone onto the sensor. The diagram of FIG. 1 is a top-down illustration and shows four such zones 4-7. These zones may, for example, be pie-slice-shaped zones that fan out from sensor 2 across a room of a home. If an infrared radiation source such as a human face is located at location A in zone 4, then infrared radiation emitted from the face will be directed by lens 3 onto sensor 2. If the person walks through the field of view of the detector from location A to location B from zone 4, to zone 5, to zone 6, to zone 7, then the electrical signal output from sensor 2 will rise and fall and rise and fall as the person moves in and out of the various zones. The resulting varying signal is referred to here as the desired signal. The desired signal is usable to detect motion of the person.
The magnitude of the variation of the desired signal is, however, small. It may, for example, rise and fall no more than one or two millivolts. This small desired signal is buried in a larger noise signal. The larger noise signal may, for example, have a peak-to-peak amplitude of ten millivolts. Fortunately, the frequency of the noise signal is generally higher than the frequency of change in the desired signal. Both the desired signal and the noise ride on a large DC offset voltage. The DC offset voltage has a large magnitude and changes with changes in environmental conditions and operational conditions. The DC offset voltage may, for example, be a DC voltage in the range of from 0.4 volts to 2.0 volts.
The circuit of FIG. 1 is a circuit such as might be found in a typical low cost home security system. The AC coupling 8 blocks the DC offset voltage from reaching the input of the gain stage 9. AC coupling 8 typically involves a fairly large capacitance such as a twenty microfarad electrolytic capacitor. Gain stage 9 may have a gain of one thousand, and is typically realized in discrete components as one or two operational amplifiers. The amplified desired and noise signals pass from gain stage 9 to a low pass filter 10. Low pass filter 10 blocks the relatively higher frequency noise signal but allows the relatively lower frequency desired signal to pass to a microcontroller 11. An analog-to-digital converter (ADC) 12 in the microcontroller digitizes the signal. ADC 12 has a resolution of approximately ten bits, and is a successive approximation type of ADC as is typically included in inexpensive microcontroller integrated circuits. The processor 13 of the microcontroller is programmed to realize a decision engine 14. Decision engine 14 analyzes the output of ADC 12 and determines whether detected changes in the desired signal should be considered to constitute movement of an object that warrant sounding an alarm.
The performance of motion detector 1 could be improved in many respects, but such improvement would generally be thought to increase the cost of the motion detector. In some motion detector markets, such as the home security system market, cost of the motion detector is extremely important. Therefore, for practical cost reasons, motion detector performance has generally not been improved for the low cost home security alarm market.